Chromiun-enriched oxide containing material and preoxidation method of making the same to mitigate corrosion and fouling associated with heat transfer components

ABSTRACT

A method of providing sulfidation corrosion resistance and corrosion induced fouling resistance for a heat transfer component is disclosed. The heat transfer component includes a heat exchange surface formed from a chromium-enriched oxide containing material formed from the composition δ, ε, and ζ, wherein ζ is a steel containing at least about 5 to about 40 wt. % chromium, ε is a chromium enriched oxide (M 3 O 4  or M 2 O 3  or mixtures thereof) formed on the surface of the steel ζ, wherein M is a metal containing at least 5 wt. % Cr based on the total weight of the metal M, and δ is a top layer formed on the surface of the chromium-enriched oxide ε, comprising sulfide, oxide, oxysulfide, and mixtures thereof. The top layer δ comprises iron sulfide (Fe 1-x S), iron oxide (Fe 3 O 4 ), iron oxysulfide, iron-chromium sulfide, iron-chromium oxide, iron-chromium oxysulfide, and mixtures thereof. The metal M of the chromium enriched oxide (M 3 O 4  or M 2 O 3  or mixtures thereof) may comprise Fe, Cr, and constituting elements of the steel ζ. The steel ζ may be selected from low chromium steels, ferritic stainless steels, martensitic stainless steels, austenitic stainless steels, duplex stainless steels and precipitation-hardenable alloys.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/641,750, filed on Dec. 20, 2006, which relates and claims priority toU.S. Provisional Patent Application No. 60/751,985, filed Dec. 21, 2005,entitled “Corrosion Resistant Material For Reduced Fouling, A HeatExchanger Having Reduced Fouling And A Method For Reducing HeatExchanger Fouling in a Refinery” and U.S. Provisional Patent ApplicationNo. 60/815,844, filed Jun. 23, 2006 entitled “A Method of Reducing HeatExchanger Fouling in a Refinery” and U.S. Provisional Patent ApplicationNo. 60/872,493, filed Dec. 4, 2006 entitled “An Insert and Method ForReducing Fouling In A Process Stream.”

FIELD OF THE INVENTION

This invention relates to the reduction of sulfidation or sulfidiccorrosion and the reduction of depositional fouling in general and inparticular the reduction of sulifidation/sulfidic corrosion and thereduction of depositional fouling in heat transfer components, whichinclude but are not limited to heat exchangers, furnaces and furnacetubes located in refining facilities and petrochemical processingfacilities and other components used for transporting or conveyingprocess streams, which may be prone to fouling. In particular, thepresent invention relates to the reduction of corrosion and foulingassociated with process streams. The present invention is directed to achromium-enriched oxide for use in the reduction of corrosion andfouling in heat transfer components. The present invention is directedto a method of reducing fouling in a heat transfer component, whichcombines the use of a corrosion resistant material having the desiredsurface roughness with the application of vibration, pulsation andinternal turbulence promoters.

BACKGROUND OF THE INVENTION

Heat transfer components are used in refinery and petrochemicalprocessing applications at various locations within the facilities toadjust the temperature (i.e., heat or cool) of the processed fluid(e.g., crude oil or derivatives thereof). The heat transfer components(e.g., a heat exchanger, furnaces, and furnace tubes) may be near thefurnace to pre-heat the temperature of the oil prior to entry into thefurnace (i.e., late-train). A typical tube-in-shell heat exchangerincludes a plurality of tubes through which the oil may flow through andaround. A hot fluid and a cold fluid enter separate chambers or tubes ofthe heat exchanger unit. The hot fluid transfers its heat to the coldfluid. The heat exchanger is designed to efficiently transfer heat fromone fluid to another. The hot and cold fluids are never combined. Heattransfer occurs through the tube wall that separates the hot and coldliquids. By employing the correct flow rate and maximizing the area ofthe partition, heat exchanger performance can be optimally controlled. Avariety of other heat exchanger designs, such as spiral heat exchangers,tube-in-tube heat exchangers and plate-and-frame heat exchangers operateessentially on the same principles.

During normal use with contact between the oil and the heat exchanger,corrosion and the build-up of deposits occurs. This build-up of depositsis often called fouling. Fouling adversely impacts the optimal controlof the heat exchanger. Fouling in this context is the unwanteddeposition of solids on the surfaces of the tubes of the heat exchanger,which leads to a loss in efficiency of the heat exchanger. Fouling isnot limited to heat exchangers. Fouling may occur in other heat transfercomponents and transfer components for transferring process fluids. Theloss in heat transfer efficiency results in higher fuel consumption atthe furnace and reduced throughput. The buildup of foulants in fluidtransfer components results in reduced throughput, higher loads onpumping devices and plugging of downstream equipment as large pieces offoulant periodically dislodge and flow downstream. As a result offouling, the transfer components and heat transfer components must beperiodically removed from service to be cleaned. This decreases overallfacility reliability due to shutdowns for maintenance. This also leadsto increased manpower requirements due to the number of cleaning crewsrequired to service fouled heat exchanger and process fluid transfertubes. Another detriment is an increase in volatile organic emissionresulting from the cleaning process.

During normal use, the surfaces of the tubes of the heat exchanger aresubject to corrosion as a result of the prolonged exposure to the streamof crude and other petroleum fractions. Corrosion on the surfaces of thetubes creates an uneven surface that can enhance fouling because thevarious particles found in the petroleum stream may attach themselves tothe roughened surface. Fouling is not limited solely to the crude oilsbeing processed. The vacuum residual streams are often used to heat thecrude within the tubes. These streams often contain solids and are highfouling. Fouling can be associated with other process streams includingbut not limited to air. Fouling can be associated with other processstreams including but not limited to process gases (e.g., air andsteam).

While the problems of fouling extend beyond petroleum refining andpetrochemical processing, the presence of crude oil presents numerousobstacles in preventing fouling that are unique to petroleum refiningand petrochemical processing not present in other industries. Crude oil,in the context of fouling, is in reality more than simply a petroleumproduct produced from an underground reservoir. Crude oil is a complexmixture of organic and inorganic components which may result in avariety of foulant deposits on the surfaces of the heat exchangerincluding but not limited to both surfaces of the heat exchanger tubes,the baffles and the tube sheets. For example, crude oil as it isreceived at the refinery often contains corrosion byproducts such asiron sulfide, which are formed by the corrosion of drilling tubulars,pipelines, tanker holds and crude storage tanks. This material, underthe right conditions, will deposit within heat exchangers resulting indepositional fouling. Crude oils often contain aqueous contaminants,some of which arrive at the refinery. Desalting is used to remove mostof this material, but some of these contaminants pass through thedesalter into the crude preheat train. These dissolved salts can alsocontribute to depositional fouling. Sodium chloride and variouscarbonate salts are typical of this type of foulant deposit. As more andmore chemicals are used to enhance production of crude from oldreservoirs, additional inorganic materials are coming to the refineriesin the crude oil and potentially contributing to fouling.

Crude oils are typically blended at the refinery, and the mixing ofcertain types of crudes can lead to another type of foulant material.The asphaltenic material that is precipitated by blending ofincompatible crudes will often lead to a predominantly organic type offouling, which with prolonged heating, will form a carbonaceous orcoke-like foulant deposit. Crude oils often also contain acidiccomponents that directly corrode the heat exchanger materials as well.Naphthenic acids will remove metal from the surface and sulfidiccomponents will cause sulfidic corrosion which forms iron sulfide. Thissulfidic scale that is formed is often referred to as sulfide inducedfouling.

Synthetic crudes are derived from processing of bitumens, shale, tarsands or extra heavy oils and are also processed in refinery operations.These synthetic crudes present additional fouling problems, as thesematerials are too heavy and contaminant laden for the typical refineryto process. The materials are often pre-treated at the production siteand then shipped to refineries as synthetic crudes. These crudes maycontain fine particulate silicaceous inorganic matter, such as in thecase of tar sands. Some may also contain reactive olefinic materialsthat are prone to forming polymeric foulant deposits within heatexchangers. As can be understood from this discussion, crude oils arecomplex mixtures capable of forming a wide-range of foulant deposittypes.

Currently, there are various techniques available for reducing foulingin refinery operations. One technique is avoiding the purchase ofhigh-fouling crudes or corrosive crudes. This, however, reduces the poolof feedstock that is potentially available to the refinery.Additionally, the crude oil can be tested to determine whether or notthe crude oil is compatible with the refinery. Again, this can reducethe feedstock potentially available to the refinery. Anti-foulant agentsmay also be added to the refinery stream. While these techniques areuseful in reducing the rate of fouling within the heat transfercomponents, fouling nay still occur under certain circumstances. Theheat exchangers must still be routinely removed from service forcleaning to remove the build-up of contaminants. Furnace tubes must betaken off-line and steam-air decoked or pigged because of foulantdeposition. Other alternative cleaning methods include the use ofmechanical devices (e.g., “SPIRELF” and “brush and basket” devices).These devices, however, have low reliability and high maintenance needs.

There is a need to significantly reduce fouling in heat transfercomponents in refinery and petrochemical processing operations that doesnot encounter the drawbacks associated with the current techniques.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to provide a heat transfercomponent that is resistant to fouling. The heat transfer component isused to either raise or lower the temperature of a process fluid orstream. The process fluid or stream is preferably crude oil based and isprocessed in a refinery or petrochemical facility. The preset invention,however, is not intended to be limited solely to the use of crude oils,other process streams are considered to be well within the scope of thepresent invention. The heat transfer component may be a heat exchanger,a furnace, furnace tubes or any other component within a refinery orpetrochemical facility that is capable of transferring heat from onemedium to another which is also susceptible to fouling including but notlimited to Crude Preheat, Coker preheat, FCC slurry bottoms, debutanizerexchanger/tower, other feed/effluent exchangers and furnace airpreheaters in refinery facilities and flare compressor components andsteam cracker/reformer tubes in petrochemical facilities. The heattransfer component contains at least one heat transfer element. It iscontemplated that the heat transfer component is a heat exchanger forheating crude oil in a refinery stream prior to the crude entering afurnace, whereby the heat exchanger is resistant to fouling. The heatexchanger may be a tube-in-shell type heat exchanger having a tubebundle located within a housing. The present invention is not intendedto be limited to tube-in-shell exchangers; rather, the present inventionhas application within other exchangers which are prone to fouling whensubject to petroleum and/or vacuum residual streams. The tube-in-shellexchanger includes a housing having a wall forming a hollow interior.The wall has an inner surface that is adjacent the hollow interior. Theheat transfer element may be a tube bundle located within hollowinterior of the housing. The crude oil is heated within the hollowinterior of the heat exchanger housing as the crude oil flows over thetube bundle. The tube bundle preferably includes a plurality of heatexchanger tubes.

In accordance with the present invention, each heat exchanger tube maybe formed from an aluminum or aluminum alloy coated carbon steel or asteel composition that is resistant to sulfidation or sulfidic corrosionand fouling. The use of aluminum or aluminum alloy coated carbon steelor a steel composition that is resistant to sulfidation and foulingsignificantly reduces fouling and corrosion, which produces numerousbenefits including an increase in heating efficiency, a reduction in theover amount of energy needed to heat the crude oil, an increase inrefinery throughput and a significant reduction in refinery downtime.

To further reduce and/or limit fouling, the heat transfer component maybe subject to a vibrational force, which results in the development of ashear motion in the liquid flowing within the heat exchanger. This shearmotion or turbulent flow within the heat transfer component limits theformation of any foulant on the surfaces of the component by reducingthe viscous boundary layer adjacent the walls of the heat transferelement. Alternatively, the fluid flowing through the heat transfercomponent may be pulsed to reduce the viscous boundary layer.

It is preferable that at least one of the interior surface of the wallof the heat transfer component and the inner and/or outer surfaces ofthe plurality of heat exchanger tubes is formed in accordance with thisinvention to have a surface roughness of less than 40 micro inches (1.1μm). Preferably, the surface roughness is less than 20 micro inches (0.5μm). More preferably, the surface roughness is less than 10 micro inches(0.25 μm). It is contemplated that both the inner and outer surfaces ofthe plurality of heat exchanger tubes may have the above-mentionedsurface roughness. Such a surface roughness significantly reducesfouling. The smooth surface within the inner diameter of the tubesreduces fouling of the petroleum stream flowing through the tubes. Thesmooth surfaces on the outer diameter of the tubes and on the innersurface of the housing will reduce fouling of the vacuum residual streamwithin the housing. It is also contemplated that the surfaces of thebaffles located within the heat exchanger and the surfaces of the tubesheets, which secure the tubes in place may also have theabove-mentioned surface roughness. Such a surface roughness wouldsignificantly reduce fouling on these components.

It is an aspect of the present invention to provide a heat transfercomponent that is resistant to corrosion and fouling. The heat transfercomponent includes a heat exchange surface formed from achromium-enriched oxide containing material formed from the compositionδ, ε, and ζ, wherein ζ is a steel containing at least about 5 to about40 wt. % chromium, ε is a chromium enriched oxide (M₃O₄ or M₂O₃ ormixtures thereof) formed on the surface of the steel ζ, wherein M is ametal containing at least 5 wt. % Cr based on the total weight of themetal M, and δ is a top layer formed on the surface of thechromium-enriched oxide ε, comprising sulfide, oxide, oxysulfide, andmixtures thereof. In accordance with an aspect of the present invention,it is preferable that the top layer δ comprises iron sulfide(Fe_(1-x)S), iron oxide (Fe₃O₄), iron oxysulfide, iron-chromium sulfide,iron-chromium oxide, iron-chromium oxysulfide, and mixtures thereof. Itis preferable that the metal M of the chromium enriched oxide (M₃O₄ orM₂O₃ or mixtures thereof) may comprise Fe, Cr, and constituting elementsof the steel ζ. The steel ζ may be selected from low chromium steels,ferritic stainless steels, martensitic stainless steels, austeniticstainless steels, duplex stainless steels and precipitation-hardenablealloys.

The present invention is also directed to a method of providingsulfidation corrosion resistance and corrosion induced foulingresistance to a heat transfer component surface. The method includesproviding a steel surface containing at least about 5 to about 40 wt. %chromium. A chromium-enriched oxide (M₃O₄ or M₂O₃ or mixtures thereof)is formed on the surface of the steel, wherein M is a metal containingat least 5 wt. % Cr based on the total weight of the metal M. A toplayer is formed on the surface of the chromium-enriched oxide,comprising sulfide, oxide, oxysulfide, and mixtures thereof.

The chromium enriched oxide (M₃O₄ or M₂O₃ or mixtures thereof) may beformed on the surface of the steel by exposing the steel to a low oxygenpartial pressure environment at a temperature of from about 300° C. to1100° C. for a time sufficient to effect the formation of a chromiumenriched oxide (M₃O₄ or M₂O₃ or mixtures thereof) on the surface of saida steel. The time sufficient to effect the formation of the chromiumenriched oxide (M₃O₄ or M₂O₃ or mixtures thereof) on the surface of thesteel may range from 1 min to 100 hrs. The low oxygen partial pressureenvironment is formed from gases selected from the group consisting CO₂,CO, CH₄, NH₃, H₂O, H₂, N₂, Ar, He and mixtures thereof. The low oxygenpartial pressure environment may be a gas mixture of CO₂ and CO. The lowoxygen partial pressure environment may be a gas mixture of H₂O and H₂.The low oxygen partial pressure environment may be pure hydrogen orargon having the dew point of the atmosphere is less than −40° C. Themethod further include imparting a vibrational force to the heattransfer component surface or feeding a fluid adjacent to the heattransfer component surface and imparting a pulsed force to the fluid.

It is another aspect of the present invention to provide a corrosionresistant barrier layer on a heat transfer component with theapplication of vibration, pulsation or other internal turbulencepromoters.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in connection with the followingdrawings in which like reference numerals designate like elements andwherein:

FIG. 1 is an example of heat exchanger having a plurality of heatexchanger tubes for use in a refinery operation;

FIG. 2 is a schematic view illustrating the various layers forming thesteel composition utilized in forming the heat transfer components inaccordance with an embodiment of the present invention;

FIG. 3 is a schematic view illustrating the various layers forming thealuminum clad carbon steel utilized in forming the heat transfercomponents in accordance with an another embodiment of the presentinvention;

FIG. 4 is a partial sectional view of a heat exchanger tube inaccordance with the present invention;

FIGS. 5 and 6 are images illustrating the fouling on a conventional heatexchanger tube after a field trial;

FIGS. 7 and 8 are images illustrating the significant reduction infouling on a heat exchanger tube in accordance with the presentinvention after a field trial;

FIG. 9 is an insert sleeve in accordance with the present invention;

FIG. 10 is a partial side view in section of a heat transfer componenthaving its inner diameter formed in accordance with the surfacedisclosed herein;

FIG. 11 is a partial side view in section of a heat transfer componenthaving its outer diameter formed in accordance with the surfacedisclosed herein;

FIG. 12 is a partial side view in section of a heat transfer componenthaving both its inner diameter and its outer diameter formed inaccordance with the surface disclosed herein;

FIG. 13 shows a cross section of a surface treated as explained withrespect to the first example herein; and,

FIG. 14 shows a cross section of a surface treated as explained withrespect to the second example herein.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will now be described in greater detail inconnection with the attached figures. FIG. 1 is a tube-in-shell heatexchanger 10, which is located upstream from a furnace (not shown) andemploys the principles of the present invention. The tube-in-shell heatexchanger 10 disclosed herein illustrates one application of the presentinvention to reduce sulfidation or sulfidic corrosion and depositionalfouling in refinery and petrochemical applications. The tube-in-shellexchanger 10 is just one heat transfer component falling under the scopeof the corrosion reduction and fouling mitigation measures in accordancewith the present invention. The principles of the present invention areintended to be used in other heat exchangers including but not limitedto spiral heat exchangers, tube-in-tube heat exchangers andplate-and-frame heat exchangers having at least one heat transferelement. The principles of the present invention are intended to beemployed in other heat transfer components including furnaces, furnacetubes and other heat transfer components which may be prone to petroleumand/or vacuum residual fouling. The heat exchanger 10 is used topre-heat crude oil in a refinery operation prior to entry into thefurnace. The heat exchanger 10 includes a housing or shell 11, whichsurrounds and forms a hollow interior 12. A bundle 13 of heat exchangertubes 14 is located within the hollow interior 12, as shown in FIG. 1.The bundle 13 includes a plurality of tubes 14. The tubes 14 may bearranged in a triangular configuration or a rectangular configuration.Other tube arrangements are contemplated and considered to be wellwithin the scope of the present invention. Each tube 14 has a generallyhollow interior 15 such that the crude oil to be heated flowsthere-through. The heating or warming fluid (e.g., vacuum residualstream) flows through the hollow interior 12 to pre-heat the crude oilstream as the stream flows through the hollow interior 15 towards thefurnace. Alternatively, it is contemplated that the crude oil may flowthrough the hollow interior 12 of the housing 11. The housing 11 and thetubes 14 are preferably formed from a steel composition. It iscontemplated that the housing 11 and the tubes 14 may be formed from thesame material. It is also contemplated that the housing 11 and the tubes14 may be formed from different materials. Typically, the tubes and thehousing are formed from a carbon or low chromium content steel.

As described above, heat exchangers are typically subject to foulingafter prolonged exposure to crude oil. The presence of fouling reducesthe performance of the heat exchanger. FIGS. 5 and 6 illustrate theeffects of fouling on the surface of a heat exchanger tube. The presenceof fouling reduces throughput and increases fuel consumption. FIGS. 5and 6 illustrate the amount of fouling present within the heat exchangertube after five months of operation. This fouling represents anapproximately 31% reduction in the heat exchanger efficiency in therefinery. The foulant contains sodium chloride, iron sulfide andcarbonaceous materials. As shown in FIGS. 5 and 6, significant amountsof pitting are present. Pitting can further exacerbate the foulingproblem.

By contrast, FIGS. 7 and 8 illustrate the reduction in fouling utilizingheat exchanger tubes 14 which embody the principles of the presentinvention. The surface cross-sections illustrated in FIGS. 7 and 8illustrate a marked reduction in fouling. These tubes were located inthe same heat exchanger and subject to the same operating conditionsover the same five month period. While the foulant present in theexchanger tube 14 also contained sodium chloride, iron sulfide andcarbonaceous material, the amount of foulant was significantly reduced.The thickness of the foulant was reduced to less than 10 microns. Thetubes having the reduced surface roughness also exhibited less pitting.The conventional tubes illustrated in FIGS. 5 and 6 exhibited a meanfoulant deposit weight density of 46 mg/cm². By contrast, the tubes 14constructed using principles in accordance with the present inventionillustrated at least 50% reduction in the mean foulant deposit weightdensity. The sample tubes exhibited a mean foulant deposit weightdensity of 22 mg/cm². Deposit weight density was determined by theNational Association of Corrosion Engineers (NACE) method TM0199-99. Thereduction in fouling shown in FIGS. 7 and 8 illustrate the benefits ofthe present invention.

The reduction in fouling may be obtained as a result of controlling thesurface roughness of the inner diameter surface and the outer diametersurface of the tubes 14 and/or the interior surface of the shell 11.Controlling the surface roughness of the inner diameter surface of thetubes mitigates the fouling of process fluid or crude oil within thetubes 14. Controlling the surface roughness of the outer diametersurface of the tubes 14 and the inner surface of the shell 11 mitigatesfouling associated with the heating fluid (e.g., vacuum residual)flowing within the hollow interior 12. In accordance with the presentinvention, at least one of the interior surface of the hollow interior12 and the surfaces of the tubes 14 has a surface roughness of less than40 micro inches (1.1 μm). Surface roughness can be measured in manyways. Industry prefers to use a skidded contact profilometer. Roughnessis routinely expressed as the arithmetic average roughness (Ra). Thearithmetic average height of roughness component of irregularities fromthe mean line is measured within the sample length L. The standardcut-off is 0.8 mm with a measuring length of 4.8 mm. This measurementconforms to ANSI/ASME B46.1 “Surface Texture-Surface Roughness, Wavinessand Lay,” which was employed in determining the surface roughness inaccordance with the present invention. A uniform surface roughness ofless than 40 micro inches (1.1 μm) produces a significant reduction infouling.

Further reductions in surface roughness are desirable. It is preferablethat the surface roughness be below 20 micro inches (0.5 μm). It is morepreferable that the surface roughness be below 10 micro inches (0.25μm). It is preferable that both the inner diameter surface and the outerdiameter surface have the described surface roughness. The desiredsurface roughness may be obtained through various techniques includingbut not limited to mechanical polishing and electro-polishing. In thesamples illustrated in FIGS. 5 and 6, the surface roughness of the tubeswas variable between 38 and 70 micro inches. The tubes in FIGS. 5 and 6were not polished. The tubes illustrated in FIGS. 7 and 8, which formthe basis for the present invention were polished to a more uniform 20micro inches (0.5 μm). This was accomplished using conventionalmechanical polishing techniques. The tubes were then electro-polished inan acidic electrolyte to produce a reflective surface having a surfaceroughness below 10 micro inches (0.25 μm). The treated tubes exhibited amarked reduction in fouling.

In accordance with the present invention, it is preferable that thetubes 14 be formed from a steel composition that is resistant tosulfidation or sulfidic corrosion and depositional fouling. The use ofsuch a steel composition significantly reduces fouling, which producesnumerous benefits including an increase in heating efficiency, areduction in the amount of energy needed to pre-heat the crude oil, anda significant reduction in refinery downtime and throughput. It ispreferable that the tubes 14 and/or the housing 11 of the pre-heatexchanger have several layers, as illustrated in FIGS. 2 and 4. Theprimary layer 21 is a steel composition containing three primarycomponents or constituents X, Y and Z. X denotes a metal that isselected from the group preferably consisting of Fe, Ni, and Co. X mayalso contain mixtures of Fe, Ni and Co. Y denotes Cr. In accordance withthe present invention, a steel composition contains Cr at least greaterthan 1 wt. % based on the combined weight of the three primaryconstituents X, Y and Z. Higher Cr contents are desirable for improvedsulfidation or sulfidic corrosion resistance. It is preferable that theCr content be higher than 5 wt. % based on the combined weight of threeprimary constituents X, Y and Z. It is more preferable that the Crcontent be higher than 10 wt. % based on the combined weight of threeprimary constituents X, Y and Z. Z is preferably an alloying element.

In accordance with the present invention, Z preferably includes at leastone alloying element selected from the group consisting of Si, Al, Mn,Ti, Zr, Hf, V, Nb, Ta, Mo, W, Sc, Y, La, Ce, Pt, Cu, Ag, Au, Ru, Rh, Ir,Ga, In, Ge, Sn, Pb, B, C, N, O, P, and S. Z may also contain mixtures ofSi, Al, Mn, Ti, Zr, Hf, V, Nb, Ta, Mo, W, Sc, Y, La, Ce, Pt, Cu, Ag, Au,Ru, Rh, Ir, Ga, In, Ge, Sn, Pb, B, C, N, O, P, and S. The weight percentof an alloying element is preferably higher than 0.01 wt. %, and morepreferably higher than 0.05 wt. %, and most preferably higher than 0.1wt. %, based on the combined weight of three primary constituents X, Yand Z. The combined weight percent of all alloying elements in a steelcomposition is preferably less than 10 wt. %, and more preferably lessthan 5 wt. %, based on the combined weight of three primary constituentsX, Y and Z. While other compositions are considered to be within thescope of the present invention, the above-described composition has beenfound to reduce fouling.

Table 1 illustrates non-limiting examples of a steel composition that isresistant to sulfidation or sulfidic corrosion and corrosion inducedfouling for use in both refining and petrochemical applications. Othermaterials exhibiting similar properties are considered to be well withinthe scope of the present invention provided such materials fall withinthe scope of the prescribed ranges.

TABLE 1 Name UNS Constituent Constituent Constituent (Grade) Number X inwt. % Y in wt. % Z in wt. % T11 K11562 Balanced Fe 1.25Cr 0.5Mo, 0.5Si,0.3Mn, 0.15C, 0.045P, 0.045S T22 K21590 Balanced Fe 2.25Cr 1.0Mo, 0.5Si,0.3Mn, 0.15C, 0.035P, 0.035S T5 S50100 Balanced Fe 5Cr 0.5Mo, 0.5Si,0.3Mn, 0.15C, 0.04P, 0.03S T9 J82090 Balanced Fe 9Cr 1.0Si, 0.35Mn,0.02C, 0.04P, 0.045S 409 S40900 Balanced Fe 10.5Cr 1.0Si, 1.0Mn, 0.5Ni,0.5Ti, 0.08C, 0.045P, 0.045S 410 S41000 Balanced Fe 11.5Cr 0.15C,0.045P, 0.03S 430 S43000 Balanced Fe 16Cr 1.0Si, 1.0Mn, 0.12C, 0.045P,0.03S XM-27 S44627 Balanced Fe 25Cr 0.5Ni, 0.75Mo, E-Brite 0.4Si, 0.4Mn,0.05Nb, 0.2Cu, 0.01C, 0.02P, 0.02S, 0.015N SeaCure S44660 Balanced Fe25Cr 1.5Ni, 2.5Mo, 1.0Si, 1.0Mn, 0.05Nb, 0.2Cu, 0.025C, 0.04P, 0.03S,0.035N 304 S30400 Bal. Fe, 8Ni 18Cr 2.0Mn, 0.75Si, 0.08C, 0.04P, 0.03S304L S30403 Bal. Fe, 8Ni 18Cr 2.0Mn, 0.75Si, 0.035C, 0.04P, 0.03S 309SS30908 Bal. Fe, 12Ni 22Cr 2.0Mn, 0.75Si, 0.75Mo, 0.08C, 0.045P, 0.03S310 S31000 Bal. Fe, 19Ni 24Cr 2.0Mn, 1.5Si, 0.75Mo, 0.25C, 0.045P, 0.03S316 S31600 Bal. Fe, 11Ni 16Cr 2.0Mn, 0.75Si, 2.0Mo, 0.08C, 0.04P, 0.03S316L S31603 Bal. Fe, 11Ni 16Cr 2.0Mn, 0.75Si, 2.0Mo, 0.035C, 0.04P,0.03S 321 S32100 Bal. Fe, 9Ni 17Cr 2.0Mn, 0.75Si, 0.4Ti, 0.08C, 0.045P,0.03S 2205 S32205 Bal. Fe: 4.5Ni 22Cr 2.0Mn, 1.0Si, 3.0Mo, 0.03C, 0.14N,0.03P, 0.02S 2507 S32507 Bal. Fe: 6Ni 24Cr 1.2Mn, 0.8Si, 3.0Mo, 0.5Cu,0.03C, 0.2N, 0.035P, 0.02S AL-6XN N08367 Bal. Fe: 24Ni 20Cr 6.2Mo,0.4Si, 0.4Mn, 0.22N, 0.2Cu, 0.02C, 0.02P, 0.03S, 0.035N Alloy N08800Bal. Fe: 30Ni 19Cr 0.15Ti, 0.15Al 800

The chromium enrichment at the surface of the non-fouling surface isadvantageous. Therefore, the steel composition preferably includes achromium enriched layer 22. The Cr-enriched layer 22 is formed on theprimary layer 21. The layer 22 may be formed on both the inner surfaceand the exterior surface of the tubes. The thickness of the Cr-enrichedlayer 22 is greater than 10 angstroms. The Cr-enriched layer 22 containsthe same three primary components or constituents X, Y and Z. X denotesa metal that is selected from the group preferably consisting of Fe, Ni,and Co. X may also contain mixtures of Fe, Ni, Co and Ti. Y denotes Cr.It is contemplated that Y may also comprise Ni, O, Al, Si and mixturesthereof. The percentage of Cr is higher in layer 22 when compared to theprimary layer 21. In accordance with the present invention, Cr contentin layer 22 is at least greater than 2 wt. % based on the combinedweight of three primary constituents X, Y and Z. It is preferable thatthe Cr content be higher than 10 wt. % based on the combined weight ofthree primary constituents X, Y and Z. It is more preferable that the Crcontent be higher than 30 wt. % based on the combined weight of threeprimary constituents X, Y and Z. In the layer 22, the ratio of Y to X isgreater than the ratio of Y to X in the layer 21. The ratio should begreater by a factor of at least 2. The ratio should preferably begreater by a factor of at least four. More preferably, the ratio shouldbe a greater by a factor of eight. Z is preferably an alloying element.

For example, 5-chrome steel (T5) nominally contains about 5 wt. %chromium per about 95 wt. % iron to give an untreated surface ratio of0.05 in the primary layer 21. In the Cr-enriched layer 22, the ratioincreased to at least 0.1, preferably to 0.2 and most preferably to 0.4chromium atoms per iron atom in the surface layer of the heat exchangertube. For 316L stainless steel, which has nominally 16 wt. % Cr, 11 wt.% Ni, 2 wt. % Mn, 2 wt. % Mo, the bulk ratio of chromium to iron wouldbe 16/69=0.23. After treatment to enrich the surface chromium, the ratiomay rise to at least 0.46, preferably 0.92 and most preferably 1.84.

In the Cr-enriched layer 22, Z preferably includes at least one alloyingelement selected from the group consisting of Si, Al, Mn, Ti, Zr, Hf, V,Nb, Ta, Mo, W, Sc, Y, La, Ce, Pt, Cu, Ag, Au, Ru, Rh, Ir, Ga, In, Ge,Sn, Pb, B, C, N, O, P, and S. The weight percent of an alloying elementis preferably higher than 0.01 wt. %, and more preferably higher than0.05 wt. %, and most preferably higher than 0.1 wt. %, based on thecombined weight of three primary constituents X, Y and Z.

It is contemplated that the Cr-enriched layer 22 may be formed on twosides of the primary layer 21 such that both the interior surface andthe exterior surface contain a Cr-enriched layer. The Cr-enriched layer22 may be formed on the primary layer 21 using one of severaltechniques. The Cr-enriched layer may be formed by electro-polishing thetube in a solution containing chromic acid. This is effective when theCr content in the steel composition is less than about 15 wt. %. It isalso contemplated that the Cr-enriched layer 22 may be formed usingvarious other formation techniques including but not limited toelectroplating chromium onto another alloy such as a carbon steel,bright annealing, passivation, thermal spray coating, laser deposition,sputtering, physical vapor deposition, chemical vapor deposition, plasmapowder welding overlay, cladding, and diffusion bonding. It is alsopossible to choose a high chromium alloy including but not limited to304L stainless steel, 316 stainless steel and AL6XN alloy. In accordancewith the present invention, the secondary layer 22 may be mechanicallypolished and/or electro-polished as described above in order to obtain auniform surface roughness of less than 40 micro inches (1.1 μm),preferably less than 20 micro inches (0.5 μm) and more preferably lessthan 10 micro inches (0.25 μm). The desired surface roughness can alsobe achieved using fine abrasive polishing or metal peening.

The Cr-enriched layer 22 may be formed on the primary layer 21 by brightannealing the tube. Bright annealing is an annealing process that iscarried out in a controlled atmosphere furnace or vacuum in order thatoxidation is reduced to a minimum and the surface remains relativelybright. The process conditions such as atmosphere, temperature, time andheating/cooling rate utilized during the bright annealing process willbe dependent on the metallurgy of the alloy being acted upon. Theskilled artisan can easily determine the conditions based on the alloy'smetallurgy. As a non-limiting example, the austenitic stainless steelsuch as 304L can be bright annealed in either pure hydrogen ordissociated ammonia, provided that the dew point of the atmosphere isless than −50° C. and the tubes, upon entering the furnace, are dry andscrupulously clean. Bright annealing temperatures usually are above1040° C. Time at temperature is often kept short to hold surface scalingto a minimum or to control grain growth.

In accordance with the present invention, a protective layer 23 ispreferably formed on the Cr-enriched layer 22. The Cr-enriched layer 22is necessary for the formation of the protective layer 23. Theprotective layer may be an oxide layer, a sulfide layer, an oxysulfidelayer or any combination thereof. The protective layer 23 preferablyincludes a material such as a magnetite, an iron-chromium spinel, achromium oxide, oxides of the same and mixtures thereof. The layer 23may also contain a mixed oxide sulfide thiospinel. While it is possibleto form the protective layer 23 on the Cr-enriched layer 22 prior toinstallation of the tubes 14 within the housing 11 of the pre-heatexchanger 10, the protective layer 23 is preferably formed on theCr-enriched layer 22 after the tubes 14 are located within the exchanger10 and the pre-heat exchanger is operational. The protective layer 23forms when the Cr-enriched layer is exposed to the process stream athigh temperatures. In a late-train heat exchanger application, theprotective layer forms at temperatures up to 400° C. In applications ina furnace or outside the late-train heat exchanger, the protective layerforms at temperatures up to 600° C. In petrochemical applicationsincluding use in steam cracker and reformer tubes, the protective layerforms at temperatures up to 1100° C. The thickness of the protectivelayer 23 is preferably greater than 100 nm, more preferably greater than500 nm, and most preferably greater than 1 micron. As illustrated inFIGS. 6 and 7, a field trial of 5-chrome steel revealed about 1 micronthick Cr-enriched magnetite layer formed during about 4 months ofperiod. Since the stream oil flowing within a heat exchanger tube is ahighly reducing and sulfidizing environment, the protective layer 23 canfurther convert to a mixed oxide-sulfide layer or a thiospinel-typesulfide layer after prolonged exposure. Applicants note that theformation of the protective layer 23 is a result of theelectro-polishing of the Cr-enriched layer 21.

The formation of the protective layer further reduces fouling. Thefoulants, which form on the protective layer 23 exhibit significantlyless adhesion characteristics when compared to foulants, which form onsurfaces that do not have the protective layer. One benefit of thisreduced adhesion lies in the cleaning of the heat exchange surface. Lesstime is required to remove any foulants from the tubes. This results ina decrease in downtime such that the pre-heat exchanger can be servicedin a more efficient manner and placed back online sooner. Also, with aless adherent deposit, on-line cleaning methods may become moreeffective or at least more rapid, which will further reduce downtime andthroughput loss.

There are numerous additional benefits of reducing the surface roughnessof the tubes 14. One of the benefits is the shifting from a lineargrowth rate of the foulant, which results in the continuous thickeningof the foulant deposit; to an asymptotic growth rate which reaches afinite thickness and then stops thickening.

The tubes 14 disclosed above may be used to form new heat exchangers.The tubes 14 can also be used in existing exchangers as replacementtubes. The use of the tubes 14 should produce significant benefits inthe refinery operations. In addition to reducing fouling, there is areduction in the number of scheduled downtimes the heat exchangersoperate more efficiently because the harmful effects of fouling arereduced. In addition, as demonstrated in the field test, the use of thetubes 14 will also prolong tube life due to reduced pitting corrosion.

The tubes 14 in accordance with the present invention may be used toretrofit an existing heat exchanger during a scheduled downtime. Theexisting tubes can be removed from the heat exchanger. The tubes 14having the above described surface roughness and/or material compositionare installed in the interior 12 of the housing 11. While it ispreferable to replace all of the existing heat exchanger tubes withreplacement tubes having the above-described construction in order tomaximize the reduction in fouling, the present invention is not intendedto be so limited. It is contemplated that only a portion of the existingheat exchanger tubes be replaced with replacement tubes. While such aconstruction may not result in the same reduction in fouling, a degreeof fouling mitigation will be obtained. The determination of the numberand location of existing tubes to be replaced by the replacement tubescan be determined by a physical inspection of the tubes within thebundle within the heat exchanger. The tubes located closest to thefurnace may be more prone to fouling. As such, it is also contemplatedthat tubes located most closely to the furnace may be replaced withtubes 14.

It may not be cost effective to replace all or a portion of the tubes ofthe heat transfer component with tubes 14 constructed in accordance withthe principles of the present invention. In accordance with anotheraspect of the present invention, an insert 50 is provided for use inretrofitting existing heat transfer components to mitigate fouling. Theinsert 50 will be described in connection with FIG. 9. The insert 50 issized such that the outer diameter of the insert is sized to abut theinner diameter surface of the tube 60. The tube 60 is secured to a tubesheet 70. Since it is intended that the insert 50 is to be retrofit inexisting operation heat transfer components, it is contemplated that alayer of corrosion 61 (e.g., an oxidized layer) may exist between thetube 60 and the insert 50. It is also possible to use the insert forfitting over a tube. In this case, the insert is sized to closelysurround the tube, which is susceptible to fouling, build-up orcorrosion on its outer diameter surface. The insert 50 in accordancewith the present invention may be constructed from any of the materialsdescribed herein having the above described surface roughnesses. It isimportant that the insert or sleeve 50 contact the tube 60 such that theheat transfer properties are not adversely or significantly diminished.

One advantage of the use of inserts 50 is that it allows one to retrofitan existing heat exchanger and convert it thereby to a non-fouling heatexchanger. This avoids the cost and time associated with theconstruction of a new bundle. For example, an existing heat exchangersubject to repeated fouling and requiring significant downtime due torequired cleaning could be retrofitted with stainless steel inserts andelectro-polished in situ to achieve a smooth, corrosion-resistant,non-fouling tube inner diameter surface. This would be much cheaper thanreplacing the entire heat exchanger bundle with a stainless steelequivalent (including tube sheets and baffles) and usingelectro-polished stainless steel tubes.

An additional advantage is that in some applications, it is not possibleto use solid tubes made of a given alloy, even if this could preventfouling on the inner diameter. As such, it is possible to use an insertwith new or replacement tubes. For example, there are real worldexamples where fouling on the tube inner diameter could be prevented byuse of an electro-polished stainless steel tube instead of the currentlyused 5-chrome tube. However, in this case, the shell side, or the tubeouter diameter, is exposed to an aqueous environment at elevatedtemperatures where stainless steel cannot be used. The potential forstress corrosion cracking is the issue. The advantage of the presentinvention is that an electro-polished stainless steel insert can be usedwithin the 5-chrome tube. The electro-polished stainless steel insertsreduces fouling on the tube inner diameter and the 5-chrome tube itselfis adequate with respect to the aqueous environment on the tube outerdiameter. Similarly, highly corrosion resistant and reduced surfaceroughness titanium alloy inserts could be inserted within a lesscorrosion resistant outer tube for use in applications where a highlycorrosive process fluid is passing though the tubes and a less corrosivefluid is in contact with the outer diameter. These are just two examplesof the generic situation where the use of electro-polished inserts maybe the only feasible route to reducing tube inner diameter fouling.

The insert 50 is relevant to fouling beyond the petroleum andpetrochemical industries. Other potential applications include forexample, black liquor fouling and paper slurry pipe systems in the paperand pulp industry, microbiological contamination (bio-fouling) in thewater treatment and distillation industry, product contamination in thepharmaceutical and semiconductor industries, reduction of contaminationpick-up and increasing the effectiveness of conventional decontaminationtechniques for recirculation piping in the nuclear power industry andpiping and exchangers used in the food, dairy and beverage industrieswhere product build-up is a problem. Polymer sheeting in chemicalreactor piping and heat exchangers used to produce polymers could alsobenefit as well as heat exchangers used to remove water duringcrystallization processes. There are many other examples of heatexchanger and piping fouling that can be ameliorated by this invention.

Many potential metal types can be used in this invention and the metalchosen will depend on the nature of the fluid stream and the type offouling that is being prevented. Though austenitic stainless steels suchas types 304 and 316 can be used to fabricate the tube inserts, otherstainless steel alloys could also be used, such as martensitic stainlesssteels such as type 410, ferritic stainless steels such as type 430.Other high performance alloys could also be considered such assuperaustenitic stainless steels such as AL-6XN, duplex steels such asalloy 2205 and superferritic grades. Nickel based alloys will also beuseful in certain environments and include the Ni—Cu, Ni—Cr—Fe, andNi—Cr—Mo—Fe families of alloys. Cobalt and titanium alloys are alsopossible as well as pure metal inserts. Any metal or alloy that can beelectropolished or otherwise smoothed is a possible material for use asan insert.

As is currently the case with installing inserts into heat exchangertubes, good metal-to-metal contacting must be insured in order tominimize heat transfer losses that could occur due to air gaps betweenthe outer tube and the tube liner. In all cases, the tube inner diametermust be as clean as possible and free of solids or liquids before theinsert is expanded. Even with cleaning, a layer of corrosion 61 mayexist. Ensuring a clean surface is relatively straight forward for newtubing, but can be more problematic for used tubing. Hydroblasting ofthe used tube, drying and light mechanical honing may be required.Following hydrostatic expansion of the insert, mechanical rolling of theends of the inserts is also required to produce a good mechanical sealbetween the insert and the outer tube. A tube would be similarlyprepared for use with an “outer” insert or sleeve.

A variation of the present invention will now be described in greaterdetail in connection with FIG. 3. FIG. 3 illustrates an aluminum oraluminum alloy coated carbon steel that may be effective in reducingcorrosion and mitigating fouling. A carbon steel layer 31 is coated orclad with an aluminum layer 32. The aluminum layer or aluminum alloy maybe applied by immersion of the steel in molten aluminum or aluminumalloy or by thermal spraying of aluminum powder or wire that isatomized. When used in a tube 14, the aluminum layer 32 is located onboth the inner diameter surface and the outer diameter surface of thetube 14 similar to the Cr-enriched layer 22.

In accordance with another aspect of the invention, the heat transfercomponent, such as heat exchange tubes or inserts, can be constructedfrom a chromium-enriched oxide containing the material having acomposition containing δ, ε, and ζ. In this case, ζ is a steelcontaining at least about 5 to about 40 wt. % chromium. ε is a layerformed on the surface of the steel ζ and is a chromium-enriched oxide(M₃O₄ or M₂O₃ or mixtures thereof) wherein the metal M contains at least5 wt. % Cr based on the total weight of the metal M. δ is a top layerformed on the surface of the chromium-enriched oxide ε comprisingsulfide, oxide, oxysulfide, and mixtures thereof.

The steel ζ contains at least about 5 to about 40 wt. % chromium and isselected from low chromium steels, ferritic stainless steels,martensitic stainless steels, austenetic stainless steels, duplexstainless steels and precipitation-hardenable alloys. A non-limitingexample of the base metal ζ is listed in Table 2, below.

Alloy Compositions Base Metal, ζ Alloy UNS No. (Weight %) Low chromiumT5 S50100 Bal. Fe: 5Cr: 0.5Mo, steels 0.5Si, 0.3Mn, 0.15C, 0.04P, 0.03ST9 J82090 Bal. Fe: 9Cr: 1.0Si, 0.35Mn, 0.02C, 0.04P, 0.045S Ferritic 409S40900 Bal. Fe: 10.5Cr: 1.0Si, stainless 1.0Mn, 0.5Ni, 0.5Ti, steels0.08C, 0.045P, 0.045S 410 S41000 Bal. Fe: 11.5Cr: 0.15C, 0.045P, 0.03S430 S43000 Bal. Fe: 16.0Cr: 1.0Si, 1.0Mn, 0.12C, 0.045P, 0.03SAustenetic 304 S30400 Bal. Fe: 8Ni: 18Cr: 2.0Mn, stainless 0.75Si,0.08C, 0.04P, steels 0.03S 310 S31000 Bal. Fe: 19Ni: 24Cr: 2.0Mn, 1.5Si,0.75Mo, 0.25C, 0.045P, 0.03S 253 S30815 Bal. Fe: 11Ni: 21Cr: 1.7Si, MA0.04Ce, 0.17N, 0.08C RA8 S30615 Bal. Fe: 14.5Ni: 18.5Cr: 5H 3.5Si:1.0Al, 0.2C Duplex 2205 S32205 Bal. Fe: 4.5Ni: 22Cr: 2.0Mn, stainless1.0Si, 3.0Mo, 0.03C, 0.14N, steels 0.03P, 0.02S 2507 S32507 Bal. Fe:6Ni: 24Cr: 1.2Mn, 0.8Si, 3.0Mo, 0.5Cu, 0.03C, 0.2N, 0.035P, 0.02S

The chromium-enriched oxide ε (M₃O₄ or M₂O₃ or mixtures thereof) can beformed on the surface of the steel ζ before use of the chromium-enrichedoxide containing material, for example in a late-train crude preheatexchanger. The metal M of the chromium enriched oxide ε (M₃O₄ or M₂O₃ ormixtures thereof) preferably contains at least 5 wt. % Cr, preferably atleast 10 wt. % Cr, based on the total weight of the metal M. The metal Mof the chromium enriched oxide ε (M₃O₄ or M₂O₃ or mixtures thereof)preferably comprises Fe, Cr, and constituting elements of the steel ζ.Constituting elements of the steel ζ contain Ni, Co, Mn, Al, Si, B, C,N, P, Y, Ce, Ti, Zr, Hf, V, Nb, Ta, Mo, W, and mixtures thereof. Themetal M of the chromium enriched oxide ε (M₃O₄ or M₂O₃ or mixturesthereof) is preferred to be Cr-rich. Excessive partitioning of Cr in thechromium enriched oxide ε (M₃O₄ or M₂O₃ or mixtures thereof) attributesto superior corrosion resistance of the steel ζ. Chromium preferablyenters to vacancy sites of the oxide lattice and effectively suppressesiron transport from the steel, a prerequisite for sustaining corrosion.

The chromium enriched oxide ε (M₃O₄ or M₂O₃ or mixtures thereof) isformed on the surface of the steel ζ by exposing the steel ζ to a lowoxygen partial pressure environment at a temperature of from about 300°C. to 1100° C. for a time sufficient to effect the formation of achromium enriched oxide (M₃O₄ or M₂O₃ or mixtures thereof) on thesurface of the steel. Thermodynamically, an oxide forms on a metalsurface when the oxygen potential in the environment is greater than theoxygen partial pressure in equilibrium with the oxide. For example, thepartial pressure of oxygen in equilibrium with Cr₂O₃ at 1000° C. isabout 10⁻²¹ atm. This implies that the formation of Cr₂O₃ is favoredthermodynamically at 1000° C. in environments with oxygen partialpotentials higher than 10⁻²¹ atm.

A low oxygen partial pressure environment can be generated from gasesselected from the group consisting of CO₂, CO, CH₄, NH₃, H₂O, H₂, N₂,Ar, He and mixtures thereof. As a non-limiting example, CO₂/CO andH₂O/H₂ gas mixtures can be used. A time sufficient to effect theformation of the chromium enriched oxide ε (M₃O₄ or M₂O₃ or mixturesthereof) on the surface of the steel ranges from 1 min to 100 hrs. Thethickness of the chromium-enriched oxide ε (M₃O₄ or M₂O₃ or mixturesthereof) ranges from at least about 1 nm to about 100 μm, preferablyfrom at least about 10 nm to about 50 μm, more preferably from at leastabout 100 nm to about 10 μm. The chromium-enriched oxide ε providessuperior corrosion and fouling resistance beneficial in heat exchangertubes in refinery applications.

The chromium enriched oxide ε (M₃O₄ or M₂O₃ or mixtures thereof) may beformed on the surface of the steel ζ by bright annealing method. Brightannealing is an annealing process that is carried out in a controlledatmosphere furnace or vacuum to provide low oxygen partial pressureenvironments in order that oxidation is reduced to a minimum and thesurface remains relatively bright. The process conditions such asatmosphere, temperature, time and heating/cooling rate utilized duringthe bright annealing process are dependent on the metallurgy of thealloy being acted upon. The skilled artisan can easily determine theconditions based on the alloy's metallurgy. As a non-limiting example,the austenitic stainless steel such as 304L can be bright annealed ineither pure hydrogen or argon or dissociated ammonia, provided that thedew point of the atmosphere is less than −40° C. and the tubes, uponentering the furnace, are dry and scrupulously clean. Bright annealingtemperatures usually are above about 1038° C. Time at temperature isoften kept short to hold surface scaling to a minimum or to controlgrain growth. Vacuum furnaces can generally achieve the best atmosphericquality for bright annealing purposes. Vacuum levels in the furnace mustbe better than 1×10⁻³ Torr. Fast cooling in vacuum furnaces is generallyachieved by back filling the chamber with argon or nitrogen and thenre-circulating this gas at high velocity through a heat exchanger toremove heat.

For the top layer δ, sulfide and oxysulfide form on the surface of thechromium-enriched oxide ε in-situ during use of the chromium-enrichedoxide containing material, in a late-train crude preheat exchanger forinstance. The temperature at which the protective layer forms varies. Ina late-train heat exchanger applications, the protective layer forms attemperatures up to 400° C. In applications in a furnace or outside thelate-train heat exchanger, the protective layer forms at temperatures upto 600° C. In petrochemical applications including use in steam crackerand reformer tubes, the protective layer forms at temperatures up to1050° C. The temperatures utilized during the formation of theprotective layer will be dependent on the metallurgy of the steel beingacted upon. The skilled artisan can easily determine the uppertemperature constraints based on the steel's metallurgy. The top layer δmay contain some impurities formed from the elements constituting thechromium enriched oxide ε and the steel ζ. Preferably, the top layer δcomprises iron sulfide (Fe_(1-x)S), iron oxide (Fe₃O₄), iron oxysulfide,iron-chromium sulfide, iron-chromium oxide, iron-chromium oxysulfide,and mixtures thereof. The thickness of the top layer δ ranges from atleast about 0.1 μm to about 50 μm, preferably from at least about 0.1 μmto about 30 μm, more preferably from at least about 0.1 μm to about 10μm.

Thus, the chromium-enriched oxide containing material so formed isutilized to construct a surface that reduces sulfidation corrosion andreduces depositional fouling, in heat exchangers for example, especiallyin late-train crude preheat exchangers. The surface has a surfaceroughness of less than 40 micro inches (1.1 μm), preferably less than 20micro inches (0.5 μm), and more preferably less than 10 micro inches(0.25 μm). The chromium-enriched oxide ε (M₃O₄ or M₂O₃ or mixturesthereof) can be formed on the inner diameter (ID), the outer diameter(OD) or both the ID and the OD of the steel ζ, depending on the need formitigating corrosion and fouling. The chromium enriched oxide ε (M₃O₄ orM₂O₃ or mixtures thereof) is formed on the surface of the steel ζ byexposing the ID, OD or both the ID and OD of the steel ζ, as describedabove.

FIG. 10 shows a heat exchange component, in this case a pipe 100, havinga base 102 formed of steel ζ with a layer 104 of chromium-enriched oxideε and top layer 106 formed as a coating δ on the chromium-enriched oxidelayer on the ID.

FIG. 11 shows a heat exchange component, in this case a pipe 100, havinga base 102 formed of steel ζ with a layer 104 of chromium-enriched oxideε and top layer 106 formed as a coating δ on the chromium-enriched oxidelayer on the OD.

FIG. 12 shows a heat exchange component, in this case a pipe 100, havinga base 102 formed of steel ζ with a layer 104 of chromium-enriched oxideε and top layer 106 formed as a coating δ on the chromium-enriched oxidelayer on both the ID and the OD.

Example 1

A first example of forming the surface is as follows. A low chromiumsteel, T5, specimen is exposed to a gas mixture containing 50% H₂O and50% He at 566° C. with a flow rate of about 500 cc/min. The specimen isheated to 566° C. at a rate of 20° C./min, held for 24 hours, and cooleddown to room temperature. After heat treatment of the specimen in a lowoxygen partial pressure environment, the specimen is characterized byScanning Electron Microscopy (SEM).

FIG. 13 depicts a surface and cross sectional images of the heat treatedspecimen. A top layer comprising magnetite (Fe₃O₄) and achromium-enriched oxide (M₃O₄) can be observed. The concentration of themetal M of a chromium enriched oxide (M₃O₄) is Balanced Fe:12.0 Cr:1.5Siin wt. %. The thickness of the top Fe₃O₄ layer is about 5 μm, and thethickness of a chromium-enriched oxide (M₃O₄) is about 4 μm.

Example 2

In a second example, the specimen as prepared in Example 1 is used forcorrosion and fouling testing. The specimen is exposed to 60:40 vol. %of a crude mix at 400° C. for 4 hours in a tubing bomb test apparatus.After testing, the specimen is cleaned in toluene and acetonesequentially and characterized by SEM.

FIG. 14 depicts a surface and cross sectional images of the testedspecimen. A top layer is comprised of iron sulfide (Fe_(1-x)S) andmagnetite (Fe₃O₄). The thickness of the top layer is about 5 μm. Achromium-enriched oxide (M₃O₄) is retained. The concentration of themetal M of a chromium enriched oxide (M₃O₄) is Balanced Fe:12.0 Cr:1.5Siin wt. %. The thickness of a chromium-enriched oxide (M₃O₄) is about 4μm.

The surface treatment can be used in any heat exchange component or withany surface susceptible to fouling or corrosion. It is also possible touse the treatment with other mechanisms for reducing fouling andcorrosion. For example, the surface treatment can be used with heatexchange components that are also subject to vibration or pulsed fluidflow, which are mechanisms used to disrupt fouling and corrosiondeposition.

In one exemplary embodiment, the surface treatment can be applied to aheat exchange component in the form of a heat exchanger tube, especiallythose formed as tube bundles retained in a heat exchanger housing. It isalso contemplated that the surface treatment can be used in replacementtubes or in tube sheaths used to cover an inner or outer diameter of acorroded or fouled tube.

It is another aspect of the present invention to combine heat transfercomponents having the above described corrosion resistant materials witheither a pulsation generating device or a vibrational generating deviceto further reduce and mitigate fouling. The devices are genericallyillustrated by reference numeral 3 in FIG. 1.

It is contemplated that the pulsation device will comprise any means forapplying liquid pressure pulsations to the tube side liquid. In thesimplest concept, the device may comprise a reciprocating pump typemechanism with a cylinder connected to the inlet/outlet conduits of theexchanger and a reciprocating piston in the cylinder to vary theinternal volume of the cylinder. As the piston moves within thecylinder, the liquid will alternately be drawn into the cylinder andthen expelled from it, creating pulsations in the conduit to which thedevice is connected. The use of a double-acting pump of this kind withone side connected to the inlet conduit and the other connected to theoutlet conduit is particularly desirable since it will create thedesired pressure pulsations in the tubes regardless of the pressure dropoccurring in the exchanger tube bundle. Variation in the frequency ofthe pulsations may be afforded by variations in the reciprocation speedof the piston and any desired variations in pulsation amplitude may beprovided by the use of a variable displacement pump, e.g. a variabledisplacement piston pump, swashplate (stationary plate) pump and itsvariations such as the wobble plate (rotary plate) pump or bent axispump.

The present invention is not intended to be limited to theabove-described pump; rather, it is contemplated that other types ofpumps may also be used as the pulsation device including diaphragm pumpsand these may be practically attractive since they offer the potentialfor activation of the diaphragm by electrical, pneumatic or directmechanical means with the movement of the diaphragm controlled toprovide the desired frequency and amplitude (by control of the extent ofdiaphragm movement). Other types may also be used but gear pumps andrelated types such as the helical rotor and multi screw pumps which givea relatively smooth (non pulsating) fluid flow are less preferred inview of the objective of introducing pulsations which disrupt theformation of the troublesome boundary layer. Other types which doproduce flow pulsations such as the lobe pump, the vane pump and thesimilar radial piston pump, are normally less preferred although theymay be able to produce sufficient pulsation for the desired purpose.Given the objective is to induce pulsations, other types of pulsator maybe used, for example, a flow interrupter which periodically interruptsthe liquid flow on the tube side. Pulsators of this type may include,for example, siren type, rotary vane pulsators in which the flowinterruption is caused by the repeated opening and closing of liquidflow passages in a stator/rotor pair, each of which has radial flowopenings which coincide with rotation of the moving rotor member. Therotor may suitably be given impetus by the use of vanes at an angle tothe direction of liquid flow, e.g. by making radial cuts in the rotordisc and bending tabs away from the plane of the disc to form the vanes.Another type is the reed valve type with spring metal vanes which coverapertures in a disc and which are opened temporarily by the pressure ofthe fluid in the tube, followed by a period when the vane snaps closeduntil fluid pressure once more forces the vane open.

In order to optimize the impact within the heat transfer component, itis preferable to locate the pulsation device close to the exchanger inorder to ensure that the pulsations are efficiently transferred to theliquid flow in the tube bundle, that is, the pulsations are not degradedby passage through intervening devices such as valves. Normally, thefrequency of the liquid pulsations will be in the range of 0.1 Hz to 20kHz. The amplitude of the pulsation as measured by the incremental flowrate through the heat exchange tubes could range from about the order ofthe normal heat exchanger flow rate at the lower end of the range ofpulsation frequencies to less than 10⁻⁶ of the normal flow rate at thehigher frequencies; because of pressure drop limitations in the heattransfer component operation and/or dissipation of higher frequencies inthe fluid, the upper limit of the pulse amplitude will decrease withincreased frequency. Thus, for example, in the lower half of thisfrequency range, the amplitude of the pulsations could be from about10-2 to about the normal flow rate and with frequencies in the upperhalf of the range, from about 10-6 to 0.1 of the normal flow ratethrough the exchanger.

It is contemplated that the vibration producing device may be any meansthat is capable of imparting a vibration force on the heat exchangerunit. The vibration producing device may be of the kind disclosed inco-pending U.S. patent Ser. No. 11/436,802. The vibration producingdevice may be externally connected to the heat exchange unit to impartcontrolled vibrational energy to the tubes of the bundle. The vibrationproducing device can take the form of any type of mechanical device thatinduces tube vibration while maintaining structural integrity of theheat exchanger. Any device capable of generating sufficient dynamicforce at selected frequencies would be suitable. The vibration producingdevice can be single device, such as an impact hammer or electromagneticshaker, or an array of devices, such as hammers, shakers orpiezoelectric stacks. An array can be spatially distributed to generatethe desired dynamic signal to achieve an optimal vibrational frequency.The vibration producing device may be placed at various locations on ornear the heat exchange unit as long as there is a mechanical link to thetubes. Sufficient vibration energy can be transferred to the tubes ofthe heat exchanger at different vibration modes. There are low and highfrequency vibration modes of tubes. For low frequency modes (typicallybelow 1000 Hz), axial excitation is more efficient at transmittingvibration energy, while at high frequency modes, transverse excitationis more efficient. The density of the vibration modes is higher at ahigh frequency range than at a low frequency range (typically below 1000Hz), and vibration energy transfer efficiency is also higher in the highfrequency range. Further, displacement of tube vibration is very smallat high frequency (>1000 Hz) and insignificant for potential damage tothe tubes.

It will be apparent to those skilled in the art that variousmodifications and/or variations may be made without departing from thescope of the present invention. While the present invention has beendescribed in the context of the heat exchanger in a refinery operation.The present invention is not intended to be so limited; rather, it iscontemplated that the desired surface roughness and materials disclosedherein may be used in other portions of a refinery operation wherefouling may be of a concern. Reducing the surface roughness of othercorrosion resistant materials such as aluminized carbon steel, titanium,electroless nickel-coated carbon steel and other corrosion resistantsurfaces are extensions of this concept as delineated below. It iscontemplated that the method of reducing fouling disclosed herein can becombined with other reduction strategies to reduce fouling. Thisincludes combining the low surface roughness and/or materialcompositions disclosed herein with vibrational, pulsation, helicalshell-side baffles and internal turbulence promoters. Thus, it isintended that the present invention covers the modifications andvariations of the method herein, provided they come within the scope ofthe appended claims and their equivalents.

1. A method of providing sulfidation corrosion resistance and corrosioninduced fouling resistance to a heat transfer component surface, themethod comprising: providing a steel surface containing at least about 5to about 40 wt. % chromium; forming a chromium-enriched oxide layer(M₃O₄ or M₂O₃ or mixtures thereof) on the surface of the steel, whereinM is a metal containing at least 5 wt. % Cr based on the total weight ofthe metal M, wherein the layer having a uniform surface roughness ofless than 40 micro inches; and, forming a surface protective layerformed on the surface of the chromium-enriched oxide, comprisingsulfide, oxide, oxysulfide, and mixtures thereof.
 2. The method of claim1, wherein the chromium enriched oxide (M₃O₄ or M₂O₃ or mixturesthereof) is formed on the surface of the steel by exposing the steel toa low oxygen partial pressure environment at a temperature of from about300° C. to 1100° C. for a time sufficient to effect the formation of achromium enriched oxide (M₃O₄ or M₂O₃ or mixtures thereof) on thesurface of said a steel.
 3. The method of claim 2, wherein the timesufficient to effect the formation of the chromium enriched oxide (M₃O₄or M₂O₃ or mixtures thereof) on the surface of the steel ranges from 1min to 100 hrs.
 4. The method of claim 2, wherein the low oxygen partialpressure environment is formed from gases selected from the groupconsisting CO₂, CO, CH₄, NH₃, H₂O, H₂, N₂, Ar, He and mixtures thereof.5. The method of claim 2, wherein the low oxygen partial pressureenvironment is a gas mixture of CO₂ and CO.
 6. The method of claim 2,wherein the low oxygen partial pressure environment is a gas mixture ofH₂O and H₂.
 7. The method of claim 2, wherein the low oxygen partialpressure environment is pure hydrogen or argon having the dew point ofthe atmosphere is less than −40° C.
 8. The method of claim 2, whereinthe low oxygen partial pressure environment is vacuum having a vacuumlevel better than 1×10⁻³ Torr.
 9. The method of claim 1, furthercomprising imparting a vibrational force to the heat transfer componentsurface.
 10. The method of claim 1, further comprising feeding a fluidadjacent to the heat transfer component surface and imparting a pulsedforce to the fluid.